The present invention relates to a method of forming a copper film, and more particularly to a method of forming a copper film, the method being capable of forming on a semiconductor substrate having large steps a copper film by vapor growth that is adapted to constituting interconnections for a variety of semiconductor device.
Accompanying the recent trend toward fabricating highly dense and finer semiconductor devices interconnections of semiconductor devices must meet the following requirements:
(1) Decrease in wiring resistance;
(2) Improvement of electromigration immunity; and
(3) Improvement of step coverage.
The requirements (1) and (2) are associated with the wiring material for interconnection, and the requirement (3) is associated with the method of forming thin films. As is well known, interconnections of semiconductor devices have heretofore been realized by using aluminum or an aluminum alloy. However, copper has an electric resistance which is as low as about two-thirds that of aluminum and further exhibits better electromigration immunity which is superior to that of aluminum or aluminum alloy. That is, when used as an interconnection material for semiconductor devices, copper exhibits superior properties to those of aluminum or an aluminum alloy and may help satisfy the above-mentioned requirements (1) and (2). As for the requirement (3) associated with the method of forming thin films, the step coverage is rather favorably accomplished by the chemical vapor deposition method or the plating method rather than the evaporation method or the sputtering method. The step coverage is not well accomplished by the evaporation method or the sputtering method because particles to be deposited impinge onto and stick on a semiconductor substrate on which a film is to be formed. Therefore, particles are deposited in small amounts on the side or bottom surfaces of the step due to a shadowing effect at the step. With the plating method and the vapor growth method, on the other hand, the reaction material diffuses into holes or grooves whereby, the product formed by the reaction on the surface of the holes or grooves is deposited on the surface, so that a high degree of step coverage is obtained. Further, comparison of the chemical vapor deposition method with the plating method indicates that the chemical vapor deposition method is favorable with regard to decreased film defects represented by impurities, stress and pinholes in the obtained film, and providing excellent film quality. By taking the above-mentioned problems into consideration, therefore, it is possible to improve performance and reliability of the semiconductor integrated circuits if a thin copper film having small electric resistance and excellent electromigration immunity is formed by the chemical vapor deposition method which features good step coverage in order to form an interconnection layer for the semiconductor devices.
Described below is a conventional technology for forming a copper film by the vapor growth method. In the conventional vapor growth method of copper, an organic compound of copper is used as a source gas. In the vapor growth method which uses an organometal compound as a starting material, however, impurities such as carbon and oxygen formed by the decomposition of the organometal compound remain in large amounts in the copper film that is obtained. Therefore, properties of the thus obtained copper film are seriously deteriorated by the impurities. Another defect is that the organic compound of copper is unstable and leaves causes a problem with regard to safety.
Vapor growth of copper has been disclosed, for example, in Appl. Phys. Letter 46, pp. 97-99, 1985.